The object of a lithographic exposure tool is to put a mask pattern on top of an existing wafer pattern. The difference between the desired position and the actual position of the mask image on the existing wafer structure is called the overlay error. Heretofore, optical verniers were used for reading the differences in position of the first pattern with respect to the second pattern. To perform this test a photosensitive layer such as photoresist was deposited on the wafer by spinning and then the resist was exposed to a pattern. Thereafter, the resist was developed and the operators had to look at same under a microscope to determine the overlay error. This was time consuming. It required a judgment call on the part of the operators, and hence different operators obtained different results.
Further, the accuracy in setting up the machine might not be limited by how accurate the machine was, but could be limited by how accurate the test processing was.
As a result, it is an object of this invention to provide a new and improved apparatus for measuring overlay error which is substantially faster and more accurate.
Heretofore, a vibration test set was used for measuring the vibration of the mask image relative to the wafer stage of the lithographic tool. A pair of masks was used. One mask was loaded into the mask stage. The other mask was loaded into the wafer stage by means of removing the wafer chuck and installing a special holder. Each of the masks had three bands; a band of 5 micron wide vertical lines and spaces; a band of 5 micron wide horizontal lines and spaces; and a transparent horizontal band separating the other two. A detector was provided for each band. In operation, when the two masks were perfectly aligned, the image of the vertical lines in the vertical band on the first mask would coincide with the lines on the second mask at the wafer stage. The associated detector would measure the light transmitted to be approximately 50 percent of that sensed by the sensor monitoring the clear band. If the second mask at the wafer stage was moved horizontally 5 microns to the right or left, the light sensed would be approximately zero. Thus, variation of this signal was used to measure relative horizontal motion. In a like manner, the horizontal line bands were used to measure vertical motion. It is noted that this system used transmission. The detector was behind the second mask located in the wafer position, and hence the carriage could not be moved or the arc image would not hit the detector. As a result, this prior art system could only do static vibration testing.
Another object of the present invention, is to provide apparatus for effecting dynamic vibration testing.